This invention relates to a process for preparing a single phase niobium activated yttrium tantalate X-ray phosphor having an M' crystal structure by a method in which the nature of the flux is critical and in which the heating and cooling schedules are critical. The phosphor has essentially no impurity phases and exhibits a brightness improvement of at least about 10% over these types of phosphors produced by prior methods.
X-ray phosphors are used in x-ray intensifying screens which are used along with photographic film to enhance the photographic image formed on the film at the same time reducing the x-ray dose on the object during medical radiographic procedures. Phosphor materials used in these intensifying screens are to be colorless single phase with a polyhedral shape of well-defined crystal morphology. Also, the phosphors have to be good x-radiation absorbers and emit the light (energy) in the spectral region to which the photographic film is sensitive. Generally, it is required that the phosphor particle size be about 4-11 micrometers in order to form a thin layer when drawn in the form of screens using certain binder solutions as media. The phosphor material also has to have a high x-ray energy absorbing property. After absorbing the x-ray energy, when exposed, the phosphor should emit photons (light) strongly in the spectral region of the film sensitivity. The efficiency of x-ray energy-to-light conversion should be intense enough to obtain undistorted and sharp film images. There are several materials of such kind but only few have good properties necessary to make them as useful materials for intensifying screen applications.
Blasse and Bril (J. Luminescence, 3,109 (1970)) describes the cathodo and photo luminescence properties of various rare-earth tantalate phosphors. These material have fergunsonite (M-type) monoclinic crystal structure. Wolten & Chase (American Minerologist, 52, 1536 (1967)) report that this type of tantalates (e.g. YTaO.sub.4 and similar rare-earth tantalates) have two polymorphs, a monoclinic (I.sub.2 Space group) structure-M at low temperature and a tetragonal (Scheelite type structure with I.sub.41/a space group) at high temperature. Crystal structure transition between these two forms occur at 1325.degree. C. in YTaO.sub.4 and is reversible. They also disclose the formation of a new polymorph of yttrium tantalate and other rare earth tantalates. This new polymorph is obtained when the tantalates are synthesized (crystallized) below the above mentioned (1325.degree. C.) transformation temperature and this polymorph has a monoclinic structure with P.sub.2/a space group which is called M' phase. M' phase can be converted to M phase by heating above 1400.degree. C. and then cooling to below the transition (1325.degree. C.) temperature
Brixner and Chen (J. Electrochemical Soc., 130 (12), 1983, 2435-43) and U.S. Pat. No. 4,225,553 describe the preparation and the crystal structure of M' phase rare earth tantalate materials and their luminescence properties. They also demonstrate that the M' phase yTaO.sub.4 is an efficient host for x-ray phosphor when activated with niobium and some rare earth ions. However, it has been found that the preparation procedure is critical to obtain a single phase M'YTaO.sub.4 with increased brightness when activated with niobium. Brixner and Chen recommend the preparation of niobium activated M' rare earth tantalate phosphor by pre-firing the component oxides TaO.sub.5, NbO.sub.5, and Ln.sub.2 O.sub.3 (Ln=La, Y, Ce, and Lu) at 1200.degree. C. for 8-10 hours. The reaction products are then milled using freon solvent as grinding fluid for about 6 hours using alumina beads as grinding medium. The resulting mixture is then either alone or with 50% by weight lithium sulfate as flux material, fired at 1250.degree. C. for 10-14 hours. This procedure is cumbersome and often results in the formation of Y.sub.3 TaO.sub.7 as an impurity phase along with M'YTaO.sub.4. This impurity phase causes a decrease in the luminescence efficiency in M'YTaO.sub.4 :Nb phosphor.